The present invention relates to a process for the selective liquid phase hydrogenation of carbonyl compounds to give the corresponding alcohols, especially of citral to give geraniol/nerol or of citronellal to give citronellol, with hydrogen in the presence of a Pt/ZnO catalyst.
The prior art discloses various hydrogenation processes for xcex1,xcex2-unsaturated carbonyl compounds. It is very difficult to obtain high selectivities for the corresponding unsaturated alcohols. The hydrogenation of citral can lead to the hydrogenation of the olefinic double bonds as well as the aldehyde group, or only of the double bond conjugated to the aldehyde group, so that as well as the unsaturated alcohols geraniol or nerol, byproducts such as citronellol or citronellal can be formed.
U.S. Pat. No. 4,100,180 describes a process for the hydrogenation of unsaturated aldehydes to give unsaturated alcohols in the presence of a PtO/Zn/Fe catalyst. PtO powder is doped with Zn and Fe (suspension catalyst). In the citral hydrogenation with a citral conversion of 70%, 3.2% citronellol is obtained. In the reaction effluents, up to 25 ppm of Fe and Zn compounds are found. If the catalyst is reused, small amounts of Fe and Zn compounds must be added. The citronellol selectivities are unsatisfactorily high and additionally the use of PtO is very expensive and makes theprocess uneconomical.
Neri et al. (J. Chem. Tech. Biotechnol. 60 (1994), 83-88) describe a selective hydrogenation process of citral over a Ptxe2x80x94Sn catalyst on activated carbon. The production of the catalyst is carried out by simultaneous impregnation of the activated carbon by Pt and Sn with the use of chlorides. The citral hydrogenation is carried out in ethanol as the solvent. The addition of tin improves the activity and also the selectivity. However, the citronellol selectivities are greater than 5%.
EP 071 787 discloses ruthenium/carbon hydrogenation catalysts, and their preparation and use for selective hydrogenation of unsaturated carbonyl compounds. The Ru/Fe/C catalyst is prepared by doping of activated carbon with ruthenium chloride hydrate, drying and mixing with iron oxide powder and reduction at 500xc2x0 C. Methanol and trimethylamine are added to the catalytic reaction to improve the selectivity. The selectivities are 2.3% for citronellol and 96.5% for geraniol/nerol at 100% conversion. However, the addition of methanol and trimethylamine makes an additional separation step during the workup of the reaction effluents necessary.
EP 422 968 describes the hydrogenation of citral with an SiO2-supported catalyst, which contains 0.1 to 10% by weight of Ir, Pt, Rh or Ru as the active component and is doped with 0.01 to 10% by weight tin, lead or germanium. The reaction is carried out in hexane. The citronellol selectivities obtained are greater than 3.5%.
In the prior art procedures for the hydrogenation of xcex1,xcex2-unsaturated carbonyl compounds the selectivity to give unsaturated alcohols is not satisfactory. Particularly during the hydrogenation of citral with high citral conversions the selectivity to give citronellol is over 2%. Since citronellol/nerol mixtures are very difficult to separate by distillation, this limits their industrial utility significantly. An advantageous catalyst shall allow citronellol selectivities below 2% even at citral conversions greater than 95%.
The prior art procedures frequently involve solvents, sometimes even with the addition of auxiliaries such as trimethylamine. This increases the cost and inconvenience of the distillative workup. Furthermore the reactor volume is increased, which considerably detracts from the economic viability of the process.
It is an object of the present invention to develop an improved process for the hydrogenation of carbonyl compounds to give the corresponding alcohols, in particular for the hydrogenation of citral to give geraniol/nerol or of citronellal to give citronellol, that at high citral conversions gives low citronellol selectivities in an economical manner.
We have found that this object is achieved by a process for selective liquid phase hydrogenation of carbonyl compounds of the general formula I 
where
R1 and R2 are identical or different and are each independently hydrogen or a saturated or a mono- or polyunsaturated straight-chain or branched, optionally substituted C1-C20-alkyl radical, an optionally substituted aryl radical or an optionally substituted heterocyclic group,
to give the corresponding alcohols of the general formula II 
where R1 and R2 are each as defined above,
in the presence of hydrogen and a Pt/ZnO catalyst.
The catalyst hydrogenates the aldehyde group of the unsaturated carbonyl compound with surprisingly high selectivity.
A singly or multiply unsaturated straight chain or branched C1-C10-alkyl radical is, unless otherwise stated, a methyl, ethyl, propyl, isopropyl, n-butyl, i-butyl, t-butyl, pentyl, hexyl, heptenyl, octyl, nonyl, decyl, 1-propenyl, 2-propenyl, 2-methyl-2-propenyl, 1-pentenyl, 1-methyl-2-pentenyl, isopropenyl, 1-butenyl, hexenyl, heptenyl, octenyl, nonenyl, or a decenyl radical or the radicals corresponding to the compounds used listed below.
A C1-C4-alkyl radical is, unless otherwise stated, a methyl, ethyl, propyl, i-propyl, butyl or t-butyl radical.
An aryl radical is a benzyl, phenyl or naphthyl radical.
A heterocyclic group is, for example, a pyridine, pyrimidine, pyridazine, pyrazine, piperazine, imidazole, furan, oxazole, isothiazole, isoxazole, 1,2,3-, 1,2,4-triazole, thiazole, thiophene or indole ring.
Substituents can be methyl, ethyl, propyl, i-propyl, butyl, t-butyl, fluorine, chlorine, bromine, iodine, nitro or amino.
Examples of useful saturated carbonyl compounds include 3,7-dimethyloctan-1-al and its isomers, tetrahydrogeranylacetone, hexahydrofarnesylacetone, 6-methylheptanone and isovaleraldehyde.
Examples of useful olefinically unsaturated carbonyl compounds include citronellal, H-geranylacetone, H-nerolidol, methyl vinyl ketone, mesityl oxide, pseudoionone, dihydrofarnesylacetone, lysmeral, methylhexenone, particularly preferably citronellal or else xcex1,xcex2-unsaturated carbonyl compounds, for example acrolein, methacrolein, crotonaldehyde, prenal, farnesal or citral, more preferably citral.
A preferred embodiment of the process involves the conversion of citral to geraniol or nerol, or of citronellal to citronellol.
The Pt/ZnO catalyst employed can be used as an unsupported catalyst or else, to improve the mechanical stability, as a supported catalyst. Suitable supporting materials include all usual supporting materials, for example, xcex3-Al2O3,xcex1-Al2O3, SiO2, activated carbon, TiO2, ZrO2, zeolites or monolithic packing structures.
However, the catalyst is preferably used as an unsupported catalyst.
The process can be carried out continuously or else batchwise in suspension or in a fixed bed. The continuous method is particularly advantageous.
The suspension or fixed bed variant may be carried out in the usual reactor designs, as described, for example, in Ullmann""s Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release.
The continuous or batchwise suspension process can be carried out as described in EP 947 943 or U.S. Pat. No. 5,939,589. The catalyst is used in both the batchwise and the continuous suspension method in finely divided form, with a particle size of less than 1 mm, preferably in the range from 1 to 100 xcexcm.
The fixed bed variant includes the use of the catalyst in extruded or spall form. Typical extruded diameters lie in the range from 1 to 5 mm, the extruded lengths in the range from 1 to 20 mm. The reactor can be operated using trickle or liquid phase methods.
The reaction is carried out in both the suspension and the fixed bed methods at atmospheric or at a pressure in the range from 1 to 200 bar, preferably 10 to 100 bar, particularly preferably 20 to 50 bar. The temperatures are in the range from 25 to 200xc2x0 C., preferably from 80 to 150xc2x0 C. The reaction can be carried out with or without a solvent. Suitable solvents include lower alcohols such as methanol, ethanol or isopropyl alcohol. Furthermore, an organic base such as trimethylamine can be used if necessary. However, the process is preferably carried out without the use of a solvent or an additional base.
The Pt/ZnO catalyst employed contains 0.1-10% by weight of Pt, preferably 2-8% by weight, the BET surface area is from 1 to 30 m2/gxe2x88x921 and the Pt particle size is from 1 to 10 nm. The platinum particles undergo partial agglomeration to give agglomerates of a size of from 10 to 100 nm.
The following examples illustrate the invention.